Expert evidence is central to many international arbitrations, particularly in construction, energy, valuation, financial and intellectual property disputes. Its effectiveness depends not only on the expert’s qualifications, but also on early appointment, clear instructions, access to appropriate data and the comparability of competing methodologies.
In July 2026, CIArb published its Report on Party-Appointed Expert Witnesses in International Arbitration: Effectiveness and Best Practice, based on 419 survey responses and contributions from regional working groups. The Report provides flexible, non-binding guidance and emphasises that expert evidence is most effective when addressed early, managed transparently and integrated into the arbitration’s case-management process.
Early Appointment of Expert Witnesses
One of the Report’s most significant findings concerns the timing of expert appointments. Seventy-one per cent of respondents considered that experts should ideally be appointed during the pre-arbitration stage, either before a claim is formally commenced or after the issues between the parties have sufficiently crystallised.
Early expert involvement can provide several practical advantages.
An expert instructed at an early stage may assist a party in assessing whether a proposed technical or financial claim is supportable, identifying weaknesses in the available evidence, reviewing the proposed method of quantification and determining whether additional documents, tests or factual evidence are required.
This may prevent a party from pleading a claim that later proves difficult to substantiate through an accepted technical or financial methodology. It may also help counsel formulate claims and defences more accurately and avoid inconsistencies between the pleaded case and the expert evidence eventually submitted.
Early involvement is especially important in complex construction and engineering disputes involving several expert disciplines. For example, a quantum expert may rely on the conclusions of delay, engineering or technical experts. If those experts are instructed at different stages without proper coordination, parts of the analysis may have to be revised or repeated.
The Report also identifies cost as a major reason for the late appointment of experts. Parties may regard early expert involvement as an unnecessary expense, particularly where settlement remains possible. However, postponing the appointment may ultimately increase costs if the expert must work within a compressed timetable, reconstruct the factual background late in the proceedings or revise an analysis after receiving further information.
A More Proactive Role For The Tribunal
The Report reflects strong support for a more proactive tribunal in relation to expert evidence.
This does not mean that tribunals should interfere with the substance of an expert’s independent opinion or assist either party in formulating its case. Rather, tribunals may improve the usefulness of expert evidence by providing early procedural direction on the scope, format, sequence and presentation of that evidence.
There is particularly strong support for tribunals asking questions after the first exchange of expert reports. This practice was supported by 95% of expert witnesses, 90% of external counsel, 83% of in-house counsel and 78% of arbitrators.
Questions raised at this stage may identify unclear assumptions, unexplained methodologies, gaps in the analysis or issues on which the competing reports are not directly comparable. Addressing these matters before the hearing may be considerably more efficient than discovering them during cross-examination.
The Report also discusses technical “teach-in” sessions. In an appropriate case, experts may explain the technical background of the dispute to the tribunal without the session constituting an evidentiary hearing. Such a process may help the tribunal understand complex terminology, technical systems or methodologies before it is required to make procedural decisions concerning the expert evidence.
Any tribunal–expert engagement must, however, be transparent and carefully structured. Direct or informal communications may create concerns regarding due process, party autonomy or the appearance of prejudgment. The Report therefore favours communications conducted with the knowledge and participation of both parties and within a clearly defined procedural framework.
Common Data Sets And Comparable Methodologies
Another important recommendation is that experts should meet at an early stage to discuss the documents, data, assumptions and methodologies on which their analyses will be based.
More than 80% of respondents in every professional category supported early expert meetings concerning common data sets and methodologies.
Opposing expert reports are sometimes difficult to compare because the experts rely on different documents, apply different assumptions or address different versions of the parties’ cases. Both reports may be technically detailed, yet fail to engage with one another in a manner that assists the tribunal.
Early expert meetings may help identify:
- the documents and data on which both experts will rely;
- the methodologies each expert proposes to apply;
- the assumptions capable of affecting the outcome;
- the matters on which the experts agree; and
- the precise reasons for any disagreement.
The purpose is not to compel experts to reach the same conclusion. Genuine professional and methodological disagreement must remain possible. The objective is to ensure that the tribunal can identify whether a difference arises from the underlying evidence, the selected methodology, factual or legal assumptions, or the exercise of professional judgment.
The Report also discusses “figures-as-figures” analysis in quantum matters. Under this approach, experts calculate alternative outcomes based on the different factual or legal scenarios that the tribunal may ultimately accept. This can be highly valuable where the tribunal agrees with one party on liability but does not accept every element of that party’s quantum case.
Joint Expert Reports
The Report identifies considerable support for joint expert reports or joint statements. Approximately three-quarters of arbitrators, expert witnesses and external counsel considered that such reports should generally become the norm in arbitrations involving expert evidence.
A properly prepared joint report can help the tribunal distinguish between issues that are genuinely disputed and matters on which the experts agree. It may narrow the scope of cross-examination, reduce duplication and make the remaining technical disagreements easier to understand.
The Report also indicates that joint reports will ordinarily be issued after the experts have submitted their individual reports. Nevertheless, the appropriate sequence should depend on the nature and complexity of the case.
In disputes involving numerous issues, extensive factual records or competing methodologies, an earlier joint process may prevent the experts from producing reports that address different questions. In a more straightforward dispute, individual reports followed by a focused joint statement may be sufficient.
Expert Independence And The Role of Counsel
The Report addresses the sensitive boundary between legitimate legal instruction and improper influence over expert evidence.
Sixty-six per cent of arbitrators considered that experts are often or very often over-instructed by counsel. The same view was expressed by 60% of external counsel and all participating in-house counsel. By contrast, 73% of expert witnesses considered that over-instruction occurs never or not often.
This divergence is one of the Report’s most notable findings.
The involvement of counsel in the expert process is both legitimate and unavoidable. Counsel must explain the issues in dispute, provide relevant instructions and ensure that the expert report responds to the party’s case. The problem arises where instructions constrain the expert’s professional judgment, exclude adverse information, dictate an outcome or transform the expert into an advocate.
The Report is particularly cautious about counsel’s involvement in joint expert reports. A weighted average of 84% of respondents favoured minimal or no counsel involvement in drafting such reports, rising to 93% among expert witnesses.
Counsel may properly provide procedural or logistical assistance and help identify the questions to be addressed. However, the substantive content of the joint report should reflect genuine expert-to-expert discussion rather than another round of submissions shaped by the parties’ legal teams.
Presenting Expert Evidence Effectively
The Report also considers how expert evidence can be presented in a manner that is accessible to the tribunal.
There is significant support for concise, word-limited summaries accompanying full expert reports. Such summaries may encourage tribunals to engage with the reports when they are submitted rather than waiting until shortly before the hearing.
However, a summary must explain more than the expert’s final figures or conclusions. It should identify the issues addressed, the principal reasoning, the assumptions used and the location of the detailed supporting analysis.
The Report does not support the universal adoption of a single standardised expert report template. Different disciplines require different forms of analysis and presentation. Nevertheless, particular elements may be standardised, including the expert’s qualifications, instructions, methodology, documents relied upon, assumptions, reservations, conclusions and declaration of independence.
Scott Schedules and similar comparative tables may also be useful. They can identify areas of agreement and disagreement and briefly state the reasons for each difference. The Report correctly observes, however, that such schedules should ordinarily supplement rather than replace the underlying expert reports. A comparative table may show where the experts disagree, but it may not provide the complete analytical foundation required to understand why.
The Hearing And Witness Conferencing
The Report does not conclude that traditional cross-examination or expert witness conferencing, sometimes referred to as “hot-tubbing,” is inherently superior in every case.
The appropriate method depends on the nature of the issues, the preparation of the tribunal, the number of experts and the relevant procedural culture.
Witness conferencing may allow the tribunal to question competing experts simultaneously and compare their approaches directly. Its effectiveness, however, depends on careful preparation, a clear agenda and prior identification of the principal areas of disagreement.
The Report also records support for short opening presentations by experts at the hearing. These presentations may allow experts to explain their methodology and principal conclusions before detailed questioning begins.
By contrast, private post-hearing meetings between a tribunal and party-appointed experts raise serious concerns. Such meetings may create questions regarding ex parte communication, new evidence, procedural fairness and enforceability. Where post-hearing clarification is required, written questions, supplementary joint statements or a reconvened hearing involving both parties will ordinarily be safer.
The Practical Benefits of The Report
The Report’s principal contribution is not the creation of a rigid new procedure. It provides tribunals and parties with a range of tools that can be adapted to the circumstances of each arbitration.
Properly applied, its recommendations may:
- improve the technical foundation of claims and defences;
- identify evidential gaps before the parties’ positions become fixed;
- reduce duplication and unnecessary expert work;
- make competing expert opinions genuinely comparable;
- assist tribunals in understanding complex technical evidence;
- reinforce the independence and credibility of experts;
- narrow the issues requiring determination; and
- reduce procedural and due process risks arising from unstructured communications.
The Report’s Appendix is particularly useful because it provides suggested language for procedural orders addressing expert communications, meetings, methodologies, common data sets and report structure.
These provisions are illustrative rather than mandatory. They must be adapted to the seat of arbitration, applicable rules, procedural tradition, value of the dispute and complexity of the expert issues.
Questions that Remain
Although the Report is comprehensive, several matters remain open for further analysis. The following observations are those of Rezvanian International and should not be attributed to CIArb.
First, the Report is primarily informed by arbitrations in which written expert reports, oral hearings and cross-examination are relatively common. The application of its recommendations in civil-law-oriented, documents-only or less adversarial proceedings may require further consideration.
Second, the Report does not substantially develop the treatment of privilege and confidentiality in relation to early expert engagement. Where an expert is appointed before arbitration to test possible claims, questions may arise concerning draft reports, preliminary conclusions, communications with counsel and documents revealing weaknesses in the party’s case. The treatment of such materials may vary significantly between legal systems.
Third, although the Report discusses the use of technology in presenting evidence, it does not establish a detailed framework for artificial intelligence, algorithmic analysis or automated tools used by experts. Future guidance may need to address disclosure of AI use, verification of outputs, reproducibility, data security and responsibility for errors.
Fourth, further guidance may be useful concerning expert conflicts and disclosures, including repeat appointments, financial relationships, fee structures and the role of junior experts or analysts who perform significant parts of the underlying work.
Finally, the procedural measures recommended by the Report may themselves increase time and cost if used mechanically. Multiple expert meetings, technical conferences, tribunal questions and joint statements will not be proportionate in every dispute. The relevant procedures should therefore be selected according to the complexity and value of the case rather than adopted automatically.
CONCLUSION
CIArb’s 2026 Report reflects an important development in international arbitration: expert evidence should not be treated merely as a report delivered shortly before the hearing.
Its effectiveness depends on decisions made much earlier, including when the expert is appointed, how the expert is instructed, what evidence is available, whether competing methodologies are comparable and how actively the tribunal engages with the expert process.
The Report’s strongest lesson is therefore procedural as much as evidentiary. Better expert evidence requires early planning, transparent roles, meaningful expert-to-expert engagement and tribunal involvement calibrated to the circumstances of the dispute.
The objective is not to make every arbitration more elaborate. It is to ensure that expert evidence is planned and managed deliberately, proportionately and in a manner that genuinely assists the tribunal in resolving the dispute.
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